1. Field of the Invention
The present invention relates to the field of micro-electronics and more specifically to light sensors associated with an integrated circuit.
2. Discussion of the Related Art
Light sensors are devices which enable turning light signal into an electric signal. Several millions of these sensors can be distributed on a surface to create an electric image comprising millions of points. Usually, a sensor is formed of a light-emitting diode and of a MOS circuit enabling collecting and processing the electric signal issued by the light-emitting diode.
The light-emitting diode may be formed of single-crystal silicon. In this case, the MOS circuit and the light-emitting diode coexist in a single-crystal silicon substrate and are located substantially in the same plane. The MOS transistors which enable collection of the electric current of the light-emitting diodes are located between the light-emitting diodes. Such an architecture is expensive since the surface area taken up in the silicon is the sum of the surface areas of the light-emitting diodes and of the MOS transistors. It has a low performance since not all the light reaches the light-emitting diodes and part of it is lost between the light-emitting diodes. The light hitting the light-emitting diodes is likely to diffuse and to disturb the operation of the MOS transistors adjacent to the light-emitting diodes.
Another solution consists of forming the light-emitting diodes above the integrated circuit comprising the MOS transistors. Such a structure is described in US patent application 2004/0135209. FIG. 1 shows a structure corresponding to this US application. An integrated circuit is formed in a substrate 1. An insulating layer 2 comprising metal interconnects is formed above the integrated circuit. The light-emitting diodes are formed above the insulating layer. The light-emitting diodes comprise a lower titanium nitride (TiN) electrode 3 and an upper indium and tin oxide (ITO) electrode 4 transparent to light. The lower electrode is in electric contact with a P-type doped amorphous silicon layer 5 located above. The upper electrode is in electric contact with an underlying N-type doped amorphous silicon layer 6. These two P- and N-type doped amorphous silicon layers are separated by a lightly-doped N-type amorphous silicon layer. Thus, the structure of FIG. 1 is formed of a vertical PN-type amorphous silicon light-emitting diode having its lower electrode connected to a region 8 of the integrated circuit by a via 9 made of a conductive material.
In this type of architecture, the space lost between the light-emitting diodes is minimum, which enables collecting a maximum light signal for a minimum occupied surface area. It is further possible to use the entire surface of the underlying silicon to achieve complex electronic functions such as, for example, the image storage and processing. The light-emitting diode, made of amorphous silicon, exhibits a chromatic response substantially equivalent to that of the human eye. Such is not the case for the light-emitting diodes formed of single-crystal silicon which are particularly sensitive to infrared wavelengths. This chromatic response enables simplifying the arrangement of the color filters in the case where a light-emitting diode matrix capable of rendering a color image is desired to be formed.
However, up to now, such architectures result in low performance for the light sensor, in particular in low light. To collect the current generated by the light-emitting diode, the light-emitting diode junction must be reverse-biased. The light signal creates carriers close to the PN junction of the light-emitting diode. Such carriers are collected in the space charge area of the reverse-biased junction and then form a photocurrent which is processed by the underlying integrated circuit. The reverse biasing of the junction also generates a leakage current, called a dark current, which is a parasitic current. In the case of the light-emitting diode of FIG. 1, this dark current is greater by several decades than that obtained in the case of a light-emitting diode formed in single-crystal silicon. Under such conditions, the current generated by the light for low lightings is on the same order of magnitude as the dark current. The sensitivity of the sensor for low lighting levels is low. This requires, in the case of patent application No. US2004/0135209, implementing specific means to compensate for this dark current (“dark reference average circuit”, FIG. 8 of the US application).